1. Field of the Invention
The present invention relates generally to tape cassettes and more particularly to a tape cassette suitable for use with, for example, a video tape recorder (VTR).
2. Description of the Prior Art
In a conventional tape cassette for a video tape recorder, in order that a tape wound between two reels within the cassette housing of the tape cassette is opposed to a rotating head upon use, the magnetic surface of the tape is partially exposed to the outside from the cassette housing. Therefore, upon non-use, a front lid is used to protect the magnetic surface of the tape exposed to the outside from the cassette housing. This front lid is constructed such that when the tape cassette is loaded into the video tape recorder, the front lid is automatically rotated to allow the rotating head and the magnetic surface of the tape to be opposed to each other. Such a front lid is molded of a synthetic resin. For example, the assignee of the present application has proposed a front lid of tape cassette as is disclosed in Japanese Laid-Open Utility Model Gazette No. 62-55285.
FIG. 1 is a perspective view of the front lid disclosed in the above-mentioned gazette. Referring to FIG. 1, the front lid 1 comprises a front plate 2 of substantially L-shape in cross section with a top plate 3 and right and left side wall plates 4, 4. The front lid 1 is formed to be substantially U-shaped on the whole. The right and left side wall plates 4, 4 have pivot shafts 5, 5 implanted on their insides. These pivot shafts 5, 5 are rotatable relative to the cassette housing. To mold the front lid 1 with such a configuration, a cut-away 6 is formed through the front plate 2 and the top plate 3 at their substantially central positions to dispose a gate. Further, a slant gate disposing surface 7 is formed within this cut-away 6 and a gate chip is disposed on the gate disposing surface 7 to perform the molding. Thus, the recess is left on the surface of the front lid 1 and the satisfactory appearance of the tape cassette is spoiled. Also, such a tape cassette is not suited to the standards of a so-called 8 mm video tape cassette which has a format made recently. In the above-mentioned 8 mm video tape cassette, as shown in FIG. 2, upon non-use, the tape exposed to the outside of the tape cassette is pinched between the front lid 1 and a back lid 8 pivoted to the front lid 1 to become rotatable in the direction shown by an arrow A - A', thus the magnetic surface thereof being protected from dust and so on. The front lid 1 of this 8 mm video tape cassette comprises the front plate 2, the top plate 3 and the right and left side wall plates 4, 4 having windows 9, 9 for detecting the tape end similarly to the tape cassette shown in FIG. 1. The portion between the front plate 2 and the top plate 3 is formed as a curvature-plane 2a. To finish the surfaces of the front plate 2, the curvature-plane 2a and the top plate 3 well, the recess 6 is not formed therethrough unlike the tape cassette shown in FIG. 1. For this reason, an injection gate portion 10 has to be formed on the inside of the left or right side wall plate 4, as shown in FIG. 2.
When the front lid 1 shown in FIG. 2 is made by means of the injection molding, the injection gate portion 10 is formed at only one position inside the left or right side wall plate 4 so that the flow of the synthetic resin is not well-balanced. Thus, when the synthetic resin is injected through the injection gate portion 10 at the side of right side wall plate 4, the pressure and temperature of the resin are different in the respective portions of the front plate 2, the top plate 3 and the left side wall plate 4 so that a sink mark is frequently produced in the left side wall 4 at the opposite side of the injection gate portion 10, thus a so-called short shot being caused easily. Hence, a flow mark occurs to lower the productivity thereof. In order to suppress the appearance of the sink mark, a large injection pressure is required. If the injection pressure is increased, "warp" in the respective portions of the front lid 1 becomes large. Further, a slider mold core has to be used as a mold core for injection-molding the front lid 1. A gate chip tends to enter a parting line plane of this mold core, which fact causes "burr" or the like.